Solid-state image pickup devices are used in, for example, image pickup apparatuses such as a digital still camera and a video camera and electronic apparatuses such as a mobile terminal apparatus having an image pickup function. As the solid-state image pickup device, there is a CMOS (Complementary Metal Oxide Semiconductor) image sensor that reads out, via a MOS transistor, charges accumulated in a photodiode as a photoelectric conversion device.
In the CMOS image sensor, it is desirable for the sensitivity to be high so that image pickup signals can be acquired even at a time of a low luminance. Moreover, since the dynamic range becomes large, it is desirable for the photodiode to be difficult to be saturated.
However, the high sensitivity and the photodiode being difficult to be saturated are in a tradeoff relationship, and it is difficult to enlarge the dynamic range while maintaining a high sensitivity.
In this regard, there is proposed a CMOS image sensor that includes both high-sensitivity pixels and low-sensitivity pixels and enlarges, while maintaining a high sensitivity by the high-sensitivity pixels, a dynamic range by the low-sensitivity pixels (see, for example, Patent Documents 1 to 3).
In the CMOS image sensor disclosed in Patent Document 1, since the aperture of the photodiodes of the high-sensitivity pixels is small, it is difficult to collect light, and the sensitivity is therefore lowered. In addition, since the capacity of the photodiodes of the high-sensitivity pixels is small, the photodiodes are apt to be saturated. If the photodiodes of the high-sensitivity pixels are saturated rapidly, image pickup signals of a poor S/N ratio obtained from the low-sensitivity pixels at the time of a low luminance are used, and an accuracy of the image pickup signals is therefore lowered.
In the CMOS image sensor disclosed in Patent Document 2, for preventing crosstalk and enhancing sensitivities, the photodiodes of the high-sensitivity pixels are set to be large, and the photodiodes of the low-sensitivity pixels are set to be small. Therefore, the low-sensitivity pixels are apt to be saturated, and the dynamic range cannot be enlarged sufficiently.
In the CMOS image sensor disclosed in Patent Document 3, since the capacity of the photodiodes of the low-sensitivity pixels is small, the low-sensitivity pixels are apt to be saturated, and the dynamic range cannot be enlarged sufficiently.
Patent Document 1: Japanese Patent Application Laid-open No. 2011-188148
Patent Document 2: Japanese Patent Application Laid-open No. 2011-129638
Patent Document 3: Japanese Patent Application Laid-open No. 2005-86082